An underwater torch works by providing a source of oxygen and a combustible substance to produce a flame. It has two hoses leading down the torch that, when activated, combine the two substances and light up, according to Cornell Center for Materials Research.
The combustible gas, which is often hydrogen or acetylene depending on the circumstances, combines with the oxygen gas and produces a flame at the tip of the torch. To maintain the flame, the torch must produce a stable sphere of gas to surround and protect it from the surrounding water because the water vapor and carbon dioxide produced by the flame are not enough to do this on their own. Under the water, the gas must be exerted at a high enough pressure to overpower the pressure of the water. Underwater torches create this protective bubble by having a number of spouts that emit compressed air around the tip of the torch. This air combats the pressure of the water and protects the flame, keeping it lit.
A diver who controls this type of torch must be quite skilled. Regular torches get oxygen from the atmosphere around them. In the Sochi 2014 Olympics, a waterproof torch was carried and passed through Lake Baikal.